1. Field of the Invention
The present invention relates to a vehicular power transmission mechanism suitable for reducing the tread of rear wheels, keeping an angle of the bend of a drive shaft so that the angle is a predetermined angle or less. In addition, the present invention relates to a vehicular power transmission mechanism which is easily applied to another vehicle and wherein the cost of which can be reduced.
2. Description of Background Art
A power transmission mechanism for a vehicle is known wherein a power transmission mechanism includes concentric right and left axles extending from differential gears to both sides of the body with rear wheels that are attached to each end of the right and left axles, for example, see Japanese Utility Model No. Sho 63-21445 (second page, FIG. 3) and Japanese Utility Model No. Sho 59-106723 (fourth and fifth pages, FIG. 3).
FIG. 23 of the present specification corresponds to FIG. 3 of Japanese Utility Model No. Sho 63-21445 which will be described below. However, please note that the reference numbers are changed as compared to the original Japanese document.
FIG. 23 is a plan view showing a power transmission mechanism of a conventional type vehicle wherein the vehicle includes rear wheels 307, 307 that are driven by transmitting the output of an engine 301 to differential gears 304 via chains 302, 303 and attaching the rear wheels 307, 307 to the respective ends of rear axles 305, 306 extending to the right and the left from the differential gears 304.
Referring to FIG. 24 in the present specification, which corresponds to FIG. 3 of Japanese Utility Model No. Sho 59-106723, will be described below. However, again reference numbers are newly allocated.
FIG. 24 is a sectional view showing a power transmission mechanism of a conventional type vehicle wherein the vehicle includes a belt automatic transmission 312 that is coupled to an engine 311, differential gears 314 housed in a rear axle 313 are coupled to the belt automatic transmission 312 via a gear and a chain, rear axles 316, 316 are attached to the right and the left of the differential gears 314 and rear wheels 317, 317 are attached to these rear axles 316, 316.
In the vehicle shown in FIG. 23, the rear axles 305, 306 extend laterally from both sides of the differential gears 304 and are coupled to the rear wheels 307, 307. In case the right and left rear wheels 307, 307 are of an independent suspension type in such arrangement, they are attached to the sides of the body, respectively, via a suspension arm so that they can be vertically moved and a drive shaft provided with a constant-velocity universal joint, for example, is used for the rear axles 305, 306.
The drive shaft can transmit driving force to the rear wheels 307, 307 even if the rear wheels 307, 307 are vertically moved. However, an angle of the bend of the constant-velocity universal joint of the drive shaft is required to be a predetermined angle or less. Therefore, when the overall length of the drive shaft is short, it is difficult to reduce the angle of the bend. To keep an angle of the bend of the drive shaft at a predetermined angle or less, the overall length of the drive shaft is required to be increased. As a result, the center distance of right and left rear wheels, that is, tread (horizontal distance between the centers of the contact surfaces of the tread of right and left tires with a road surface) is increased, the width of the body is increased and the realization of keeping the angle of the bend at a predetermined angle or less is difficult in a small-sized vehicle. It damages the mobility of the vehicle to keep the angle of the bend at a predetermined angle or less. It is also similar in the vehicle shown in FIG. 24.
In addition, a vehicular power transmission mechanism in which a transmission and a differential mechanism are housed in a crankcase of an engine is known, for example, as set forth in Japanese Published Examined Patent Application No. Hei5-39998 (second page, FIG. 2).
Referring to FIG. 25 of the present specification, which corresponds to FIG. 2 of Japanese Published Examined Patent Application No. Hei5-39998, a description is set forth below. Note, that the reference numbers used for FIG. 25 are newly allocated as compared to Japanese Published Examined Patent Application No. Hei5-39998.
FIG. 25 is a sectional view showing a power transmission mechanism of a conventional type vehicle wherein a transmission mechanism 304 is coupled to a crankshaft 302 of an engine 301 via a V belt 303. A differential mechanism 306 is coupled to the transmission mechanism 304, left and right driving shafts 307, 308 are coupled to the differential mechanism 306, a rear wheel (not shown) is coupled to the driving shafts 307, 308 and the transmission mechanism 304 and the differential mechanism 306 are housed in the crankcase 311 of the engine 301.
In the above-mentioned prior art, as the transmission mechanism 304 and the differential mechanism 306 are housed in the crankcase 311 of the engine 301, that is, a case for housing the transmission mechanism 304 and the differential mechanism 306 and the crankcase 311 are integrated, a degree of the freedom of changes decreases when the gear ratio of the transmission mechanism 304 is maintained and the specifications (a cylinder bore diameter and a piston stroke) of the engine are to be changed or when the specifications of the engine 301 are maintained and the gear ratio of the transmission mechanism 304 is to be changed (the diameter of a transmission gear is changed). In the case when the power transmission mechanism is applied in another vehicle and applicable vehicles are limited. As a result, the power transmission mechanism is substantially dedicated and it is difficult to reduce the cost.